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Static Electricity
Introduction E ---- Background/Fundamentals Before delving into static electricity some important terms must be defined and understood: Protons A proton is a positive (+1) charge its specific charge is a positive elementary charge > Electrons An electron is a negative (-1) charge its specific charge is a negative elementary charge the definitions of protons, electrons and charge have been established, the formula for charge can be introduced. The formula for charge is Q = ne. In the equation Q stands for charge of an object in coulombs, n is the number of elementary charges, and e is the elementary charge (found on NYS Physics Reference Tables, to be 1.60 x 10 -19 . See this equation in action: A person has received a charge of 8 coulombs how many elementary charges does the person have? *'Q=ne' First Butthole equation **'8 C= n(1.60 x 10 -19 C)' Next plug in the given 8 C and plug in elementary charge e (found on NYS Physics Reference Table) **'5.0 x 10 19 C = n' Now divde the given Q by the elementary charge e. Now we know that there are 5.0 x 10 19 elementary charges! Goodbye,shitfaces ---- How Are Objects Charged? Before learning how to charge an object, it is important to understand the objects used in charging an object. Usually in a situation in which an object is going to be charged both a neutral object and an object that possesses a charge are needed. It is very important to note that the charged object confines a positive or negative charge. This important requirement is actually given a name, an insulator. Insulators are materials that hold the charge they are given. So if a person wanted to charge an object negatively, the process of charging by conduction could be used. First a negatively charged insulator, possibly a glass rod, would slowly come into contact with the neutrally charged object. As the rod gets closer, the positive and negative charges will start to detach and the positive charges will be attracted to the negative charges in the rod, this is called redistribution. When the rod comes into full contact, an exchange of electrons will occur, and the once neutrally charged object will have an excess of electrons making the object charge negative. This process is depicted in the figure to the right. There are two other ways to charge an object, redistribution and grounding. Redistribution is a method similar to charging by conduction except the two objects never touch, as a result the charging is only temporary. Because the two objects never touch there is essentially no transfer, the only event that takes place is the charges temporarily separating in the neutrally charged object. Grounding, also known as induction, is a method quite different to the previous two. In grounding the objects are moved closed together so the charges separate, as in redistribution, but then the neutral object is 'grounded' which means being attached to the ground, or any large neutral object such as the Earth, after being grounded the electrons will leave the object and the object will have deficiency of electrons resulting in a positive charge. How to Detect Charge So the process of charging an object has been established, but how is this charge detected? Charge is not a visible quantity, so scientist cannot measure it using the naked eye– rather an instrument called an electroscope is what detects charge. A diagram of an electroscope is provided to the right. The plate at the top of the electroscope is what detects the charge. The line moving is called the leaf. In the rested upright position there is no charge detected. In the rotated position a charge is detected. An electroscope alone will not reveal the charge of an object. An electroscope in conjunction with a known charge will produce results, which are readable in terms of positive and negative, rather than charge or no charge. Measuring the Forces Acting on Charges We now know the properties of charges, how to charge an object, and how charges are detected, but we still do not know how to measure charges interactions. When two charges interact, there are two possibilities the charges can either attract or repel (as depicted in the fundamentals section). In both of these possibilities, the charges feel either an attractive or repulsive force– we know that this force exists, but how can we measure it? Charles-Augustin de Coulomb asked this same question in 1784. Coulomb was a great French physicist who was responsible for discovering a relationship between the force of two charges and the square of its distance. He compiled this relationship into an equation called Coulomb's Law. The equation is http://upload.wikimedia.org/math/d/4/0/d400ebe3e561448fd8fff5c6608599d1.png. In this equation F stands for the total force exerted, q 1 is the charge on one of the bodies, q 2 is the charge on the other body, k is the electrostatic constant (Found on the NYS Physics Reference Table) which is 9.0 x 10 9 n x m 2 / C 2 , and r is the distance between the two bodies. Electric Fields To understand the concept of electric fields an understanding of electric force and charge is a prerequisite. An electric field is created when an electric force, the F constant in Coulomb's law, is exerted on to a charged particle. A British Physicist and Chemist named Michael Faraday discovered the concept of an electric field. Predecessors to Faraday believed that charges interacted with each other directly meaning charge A exerted a force directly and exclusively on charge B, this ideology was called action at a distance. Faraday and later scientist saw a problem with this theory, how did charge A realize charge B is in its vicinity? There is no way for charge A to know charge B is close in its immediate area, thus the action at a distance theory was invalidated. After disproving action at a distance, the idea of electric fields was introduced. The electric field model establishes that charges exert a force around a charge, when interacting with another charge, instead of in one direction. Diagramming Electric Fields Just as charges cannot be seen with the eye, electric fields are also not visible. To diagram electric fields, we use electric field lines. Electric field lines are drawn out of or into a charge. Each line represents the path a test charge (a very small positive charge) would take if it came into contact with the given charge. When multiple lines are drawn, an electric field diagram is constructed. Also the number of lines and there spacing are not drawn in a random or artistic manner instead they are representative. The number of lines drawn around a given charge represents the magnitude of that charge while the space of the drawn lines represents the approximate strength of the electric field. So a diagram with many lines drawn close together would represent a powerful charge with a strong electric field. Measuring the Strength of Electric Fields Measuring an electric field is a quite simple process involving a test charge. To measure the strength of an electric field, first a test charge must be placed in its vicinity, then calculate the force the test charge feels. The resulting number is the strength of the electric field. This process is simplified into the following equation In this equation, F is the magnitude of the force, as found by using Coulomb's Law, q is the magnitude of the test charge. The resulting electric strength is measured in Newton’s per a Coulomb. Measuring Potential Difference (Voltage) Electric Strength can also be defined in units of work and energy. This alternative way of measuring the strength of an electric field entails finding the potential difference, which exits between any two points. To visualizing this difference picture an electric field, then trying to force a test charge in between two points, if the test charge is repelled, we have to do work to push it into place. This process is also simplified into the formula In this formula, W is work, q is the magnitude of charge, and V is voltage or potential difference. Real World Applications/Occurrences Now that the basics of static electricity have been laid out, when and how does static electricity appear in real world situations? The following situations are either common occurrences or experiments done with static electricity. 1. When people say static electricity, most don't think of electrostatics rather they think of things like static shocks. In fact these two are related! After rubbing against a carpet and touching someone the other person will feel an electric shock. This occurrence is due to objects with a high voltage, high enough that they produce visible electrical sparks or attractions. 2. The Millikan Oil Drop Experiment is an experiment conducted by Robert Millikan which measures charges on microscopic oil drops. 3. When balloons are rubbed on a carpet then stuck to a wall, static electricity is to blame! If the balloon has positive or negative charge then is put next to the wall, which has a positive charge, the basic principle of attraction comes into play and the balloon stick. 4. Many don’t know that even bees use static electricity. The charge on the bee’s body is responsible for collecting and holding pollen. Without static electricity bees wouldn’t be able to pollinate Practice Problems The following practice problems are broken down by which section they pertain to in this wikipage. These questions are a mix of New York State Regents problems and other testing institutions/boards. The specific testing service is specified on each question Practice with Charging Objects 1. Regents A glass rod becomes positively charged when rubbed with silk. The silk becomes charged because it a. loses protons b. loses electrons c. gains protons d. gains electrons 2. Review, not covered on NYS Regents A charged body may cause the temporary redistribution of charge on another body without coming into contact with it. This process is called a. conduction b. potential c. permeability d. induction 3. Regents} A neutral rubber rod is rubbed with fur and acquires a charge of -2 x 10 -6 coulomb. The charge on the fur is? a. 1 x 10 -6 C b. 2 x 10 -6 C c. -1 x 10 -6 C d. -2 x 10 -6 C 4. [NYS Regents A rod is rubbed with wool. Immediatley after the rod and wool have been seperated, the net charge of the rod-wool system a. decreases b. increases c. remains the same Solutions For Charging Objects Problems Practice with Electric Fields 1. Regents What is the magnitude of the electric field intensity at a point in space where a charge of 100 coulombs experiences a force with a magnitude of 10 newtons. a. 1 N/C b. 10 N/C c. 0.1 N/C d. 100 N/C 2. Regents The work required to move 2 coulombs of charge through a potential diference of 5 volts is a. 10J b. 2J c. 25J d. 50J 3.Regents If 8.0 Joules of work is required to transfer 4.0 coulombs of charge between two points, the potential difference between the two points is a. 6.4 V b. 2.0 V c. 32 V d. 40V 4. Regents If 1.6 x 10 -12 joules of energy is needed to move a charge through a potential difference of 1 x 10 7 volts then the magnitude of the charge is a. 1.6 x 10 -19 b. 1.6 x 10 -5 c. 1.6 x 10 5 d. 1.6 x 10 19 5. II Physics} A particle of charge +2q exerts a force F on a particle of charge –q. What is the force exerted by the particle of charge –q on the particle of charge +2q? a. 1/2F b. 0 c. 2F d. F e. -F 6. [SAT II Physics Two charged particles exert a force of magnitude F on one another. If the distance between them is doubled and the charge of one of the particles is doubled, what is the new force acting between them? a. 1/4 F b. 1/2 F c. F d. 2F e. 4F Solutions For Electric Field Problems References The following links were referenced during the creation of this wiki page: 1. Barron's Regents Review Book 2. Electric Field Strength 3. Electrostatics 4. Coulomb's Law 5. Glencoe Physics: Principles and Problems 6. Introduction to Electrostatics 7. Notes from GK's Physics class on Static Electricity 8. SAT II Physics Prep Resources Look to the following websites for further information on static electricity: 1. www.school-for-champions.com Overview of electrostatics with intresting images. 2. www.enchantedlearning.comBasic easy to understand definition of static electricity and experiments. 3. Lessons On Electrostatics This website serves as an essential review of all static electricity concepts. 4. www.regentsprep.org This is a regents prep website that has an in depth explantion of all aspects of regents style electrostatics. 5. Actual Physics Regents Test The best way to learn is to practice! This link includes actual regents tests and solutions. 6. Regents Review Live This website contains a TV show that was produced to help students study for the Physics Regents.